Birds have developed an ability that any student who's ever pulled an all-nighter would envy: the ability to let half their brain sleep while keeping the other half awake. In tomorrow's issue of Nature, researchers report that ducks manage this trick to stay alert for predators while still getting some shut-eye. The findings suggest that different parts of the vertebrate brain can sleep independently of each other.

Although researchers have known about this so-called "unihemispheric" sleep for 3 decades, they could only guess why this peculiar kind of shut-eye evolved in birds. But when filming mallards for a different experiment, sleep researcher Niels Rattenborg of Indiana State University noticed a remarkable pattern. "When I put a camera on them, I noticed that ducks that were next to each other slept with the outside eye open," Rattenborg says. "From there, the experiment designed itself."

Rattenborg, along with animal behavior experts Steven Lima and Charles Amlaner, placed four ducks in a row of clear tanks and waited for them to doze off. The ducks in the middle tanks would almost always sleep with both eyes shut, while those on the ends kept one eye open for about a third of the night. When the researchers rotated the ducks to different spots, the bird that had slept with its left eye open on one end, for example, would sleep with both eyes closed in an interior tank and with the right eye open in the tank at the other end. The side of the brain that controlled the open eye had the activity levels of an awake bird, while the other side had brain waves characteristic of sleep, according to electroencephalogram recordings. Even with just one eye open, the birds reacted to a slide of a predator in less than a fifth of a second.

Apparently, the sleep pattern is as important as the amount, says Jerome Siegel, a sleep researcher at the University of California, Los Angeles. "There's a tremendous difference in the way animals sleep," he says. Each animal has evolved its own way to balance the need for sleep with the need for safety: Rats hide in nests, horses slumber lightly out in the open. Yet every animal needs to get its rest, one way or another. "Our results clearly emphasize how important sleep is," says Rattenborg. What no one knows yet, says Siegel, is why we need to sleep.